1. Field of the Invention
The present invention relates in general to a self-calibrating radiometer for non-contact measurement of the radiant temperature of an object.
2. Description of the Background Art
The measurement of radiant temperature is important to many science and industrial applications. Radiant temperature is the temperature of an object inferred from the gray body emission of the surface under study. For many problems, it is desirable to measure radiant temperature with an accuracy and precision better than 0.1K. In practice, it is difficult to routinely achieve this level of accuracy or precision with commonly used techniques (i.e. without routine calibration). An interesting and important remote sensing application requiring this level of accuracy is global warming assessments using sea surface temperatures inferred from the radiant temperature. Conversely, water bodies, such as lakes, reservoirs and others with in-situ surface monitoring temperatures, can be used to calibrate airborne and satellite infrared remote sensing systems. Water and other opaque liquids can also be useful for calibrating infrared instruments in the laboratory. In cases where a large uniform surface is needed, a liquid such as water can make an excellent calibration source.
The following characteristics for water surfaces are important in the discussions that follow. Water is 99% absorbing at 100 microns thick in the 8-12 micron spectral region; the emissivity of pure water is approximately 0.986 across the 8-12 micron spectral region; and the skin surface temperature can be different from bulk temperature by as much as 1K. Since the effective water thickness is only a fraction of a millimeter and its surface temperature can be significantly different from the bulk temperature, it is difficult to use contact measurement techniques to infer the surface temperature. This limitation can be minimized in some cases with significant stirring or agitation of the liquid""s surface, if practical. However, a more desirable solution is to employ a non-contact radiant temperature measuring technique.
Non-contact methods using infrared radiometer techniques, such as Long Wave Infrared (LWIR) 8-12 micron region, are among the best techniques for determining the skin surface temperature of remotely sensed surfaces and water bodies. These techniques, however, are usually accurate to about 1K because of drifts in the radiometer and its electronics. The accuracy can be improved with routine measurements against a known temperature source, such as a black body. Black bodies with 0.1K or better temperature accuracy and precision can be achieved by many different approaches. In general, one needs two black body temperature calibrations covering the desired temperature range to account for both offset and gain drifts. Even more sources may be necessary if the detection system is nonlinear. However, most radiometers do not have built-in calibration sources or other methods to correct for drifts in offsets and gains. If they do, it is usually only one source, and thus only accounts for offsets. This is usually because black bodies are expensive and can draw many watts. For field portable instruments, the added expense and power demands are not very desirable. Commercially available radiometers are also typically designed to provide temperature updates every few seconds, which is overkill for many objects with large thermal inertia. For example, for water and liquid bodies with long thermal time constants, temperature measurements need to be updated only once every few minutes.
In the microwave region, self-calibrating radiometers have been constructed for nearly 50 years. The operational principle of such a device is as follows. An antenna collects microwave radiation from a source under study. A detector is synchronously switched between the signal from the antenna and a known RF or microwave source referred to as the noise source. This noise source can be generated artificially or through a natural process, such as Johnson noise. If Johnson noise of a resistance is used, the temperature of the resistor can be adjusted to change its resistance and null the signal. The signal strength can then be defined in terms of the temperature of the resistor, which is then a measure of the radiant temperature of the source, if the antenna properties are well understood. This switching process produces an alternating current (AC) signal. The AC signal offers significant advantages over direct current (DC) measurements because it can be averaged for extended periods to produce a high signal-to-noise ratio (SNR). In contrast, DC signals typically cannot be averaged over extended periods due to 1/f noise and other low frequency drifts. The AC signal is synchronously rectified and integrated to produce a DC signal that is proportional to the difference between the signal detected by the antenna and that of the noise source. This difference signal is then used in a feedback loop to adjust the noise source until it is approximately equal to the signal from the antenna, which is referred to as the null condition. A voltage proportional to the noise source strength is then used to define the antenna signal.
The elegance of this approach is that one calibration source cancels both offset and gain drifts. Although this technique works well in the microwave and RF, it will not work for the infrared or any other spectral region where antenna and standard RF techniques do not work. While similar switching techniques have been employed with infrared detection systems, the switching has been done at very low frequencies, and non-continuously. For example, an infrared radiating surface will be observed for a minute and then the detector will be switched or moved so that a black body will be observed for a minute. The black body is then adjusted to equal the surface radiation. This however creates significant data gaps during the switching between the surface and the black body, and requires some means to prevent detection of radiation from sources other than the black body and the test source during switching. A need therefore remains for a low power, self-calibrating infrared radiometer that can be used for making radiant temperature measurements of objects, such as water bodies, and provide a measurement accuracy and precision on the order of 0.1K or better.
The present invention fulfills the foregoing need through provision of a radiometer that employs a black body source as a temperature reference, an optomechanical mechanism, e.g., a chopper, to switch back and forth quickly and contiguously between measuring the temperature of the black body source and that of a test source or object, and an infrared detection technique. More particularly, the radiometer functions by measuring infrared radiance of both the test and the reference black body sources; adjusting the temperature of the reference black body so that its radiance is equivalent to the test source; and, measuring the temperature of the reference black body at this point to determine the radiative temperature of the test source.
To achieve this functionality, the radiation from the reference black body source and the test object or source is detected by an infrared detector that converts the detected radiation to an AC electrical signal. The chopper is positioned between the two radiation sources and the infrared detector, and, through a movable set of optics, alternates back and forth between exposing the detector first to only a first of the two sources, second to both of the sources, and third, to only the second of the two sources. In this manner, the chopper provides continuous radiation to the detector so that the radiometer can thereby generate a continuous stream of measurement data. A reference signal that monitors the motion of the chopper is sent along with the AC detector signal to an error signal generator that can be a synchronous detector, such as a Lockin amplifier. The synchronous detector creates a precision rectified error signal that is approximately proportional to the difference between the temperature of the reference black body and that of the test object or source.
The error signal is used in a feedback loop that includes a temperature-modifying device, such as a thermoelectric cooler, to adjust the reference black body temperature until it equals that of the test source, at which point the error signal is nulled to zero. A precision temperature monitor measures the reference black body temperature at this null point, which is then an accurate estimate of the radiant temperature of the test object or source under study.
The advantages of the radiometer design include high accuracy, low power, self-calibration and nonlinearity compensation with a single reference black body source. Regarding the accuracy, since contact type temperature sensors with long-term stability and accuracy better than 0.1K are routinely available, the same level of accuracy can be achieved with the present invention since the radiant temperature of the test source is estimated from a contact temperature measurement of the reference black body source. Further, the feedback loop minimizes any nonlinearities, offset and gain drifts in the infrared detection process so that additional reference black bodies are not required to compensate for these nonlinearities. The use of the low power chopping mechanism results in the radiometer having extremely low power requirements in cases of near ambient temperature measurements. The periodic switching at one or more Hz between the reference and test sources produces a periodic signal that can be integrated with little impact of 1/f noise, enabling the use of room temperature detectors to achieve Noise Equivalent Delta Temperature (NEDT) of a few mk with several seconds of integration. Additionally, switching at these frequencies allows near continuous measurements of the test source, while a pure chopping arrangement of the chopper, discussed in further detail below, eliminates any data gaps between test source and reference black body source measurements. Finally, the radiometer is self-calibrating through contact thermometer calibration standards.
In a preferred embodiment of the invention, the chopper is implemented using an electromechanical resonator to oscillate a prism shaped reflector. The desired features of the chopper include pure chopping, low power (few mw), high reliability (millions of cycles) and one Hz or greater chopping frequency. Pure chopping means that the infrared radiation that the detector sees comes only from the reference black body source and the test source under study. It is also desirable that during part of the chopping cycle, the detector system sees only the reference black body source and during another part of the chopping cycle, the detector system sees only the test source under study. Typically, choppers block the radiation, but this is not desirable in the present invention since the object that does the blocking itself will radiate into the detector system, thus inducing an unwanted signal.
The resonator achieves the foregoing goals through use of an electromagnet assembly that drives a pair of metal leaf springs back and forth, thus also causing the reflector, which is attached to the leaf springs, to oscillate. As the reflector oscillates, the two angled reflector surfaces direct radiation received from the test source and the reference black body to the infrared detector. At the two extremes of the oscillatory motion, only radiation from one or the other of the two sources is reflected toward the detector. At the mid-point of the motion, radiation is received by the detector from both of the sources. Since the chopper is implemented by an electromechanical resonator, it behaves like a crystal watch using minimal power. The use of steel or another shim material in the leaf-springs to form a high Q resonator will allow the resonator system to oscillate with less than a few mw of power. Keeping,the shim material within the elastic limit of the material will allow the system to oscillate potentially millions of cycles without failure.